Cathodes



Aug. 6, 1957 H. c. NEDDERMAN CATHODES Filed July 12, 1951 v INVENTOR v /fibllldl'd ajldaez'mazz Ill l l A ORNEY United States Patent CATHODES Howard C. Nedderman, Orangeburg, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application July 12, 1951, Serial No. 236,361

8 Claims. (Cl. 313-=-32) The present invention relates to thermionic cathodes for electron tubes, and particularly, to magnetron cathodes.

As with most kinds of vacuum tubes, the life of a magnetron is determined essentially by cathode life. A magnetron consists essentially of a central elongated thermionic cathode surrounded by an anode structure, which is usually of the multi-cavity resonator type, and means for establishing a constant magnetic field transverse to the paths of electrons from the cathode to the anode. In the operation of the tube, an accelerating direct-current field is set up between the cathode and anode. Electrons from the cathode travel toward the anode along curved paths, due to the transverse magnetic field. When the direct-current and magnetic fields are suitably chosenor adjusted, the electron space charge revolves around the cathode-anode space and interacts with the anodeelements and resonators to generate high frequency oscillations. A large proportion of the total number of electrons are not collected by the anode, but instead, return along curved paths to the cathode. This back-bombardment of the cathode greatly intensifies the problem of cathode life. In well-designed magnetrons, the back-bombardment power loss is generally no more than about five percentof the input power; it may be considerably greater in case of improper loading. This backbombardment of the cathode causes over-heating, often with local hot spots, and contributes to sputtering with eventual loss of emitter material. Such cathodes have lives of no more than a few hundred hours.

Many attempts have been made to solve this problem. In the all-ceramic cathode, for example a sintered thoria type cathode, the emitter material is sufficiently thick and infusible to withstand bombardment, but the cathode is relatively delicate mechanically. In the water-cooled secondary-emitter cathode there is little or no control over emission.

The object of the present invention is to provide an improved cathode especially adapted for use with long life in high-power, continuous-Wave or pulsed magnetrons.

In accordance with the invention, a series or row of elongated thermionic emitters are mounted in longitudinal grooves'in the periphery of a generally-cylindrical electron-opaque shield element, with the rib portions of the shield element between the grooves shielding the emitters from back-bombardment by returning electrons. The emitters are spaced from the rib portions to permit cooling of the latter. Means are provided for cooling the shield element to dissipate the heat generated by backbombardment.

The invention will be more fully explained in the following detailed description, taken in connection with the accompanying drawing, in which Fig. 1 is a side elevational view, partly in axial section, of a magnetron cathode embodying the invention,

Fig. 2 is a sectional view, taken on the line 22 of Fig. 1; and

Fig. 3 is a fragmentary view, taken on the line 3-3 of Fig. 1.

Referring to the drawing, the numeral 1 represents a magnetron cathode comprising a generally-cylindrical central portion 3 and two hat or shield portions 5 at the ends thereof. The cathode 1 is adapted to be supported by a tubular support member 9, and other means, not shown, centrally in a multi-cavity magnetron, the anode elements 7 of which are shown in dot-dash lines.

The central portion 3 includes a tubular shield element 10 of circular cross-section having a series of parallel longitudinal grooves 11 of V-shaped cross-section uniformly distributed around the periphery thereof. In the example illustrated there are twelve grooves, separated by ribs 12. Preferably, the grooves 11 are formed with one side face of each groove disposed at an angle of about 30 with respect to a plane extending through the vertex of the groove and the central axis of the cathode, that is, perpendicular to the cylindrical surface of the shield element, and with the other side face disposed at an angle of about 60 to said plane, as indicated at the right side of Fig. 2. A plurality of elongated thermionic emitters 13, one for each groove, are disposed within the grooves 11, spaced from the walls thereof and closely adjacent to one side wall, which is the same side wall for all of the grooves as shown in Fig. 2. The major portion of each emitter 13 is substantially closer to this same side wall than the other side wall of the groove, to provide maximum shielding of the emitters by the ribs 12. As shown in Fig. 2, the ribs 12 are disposed in the paths of electrons returning to the emitters in the transverse magnetic field at angles from Zero up to about 60 to the periphery of the shield 10, or 30 to the radial direction. The emitters 13 may be directly-heated filaments of fiat ribbon shape coated with electron emitting material. Alternatively, dispenser-type emitters in the form of perforated trough elements packed with thoria powder and sintered in place may be used.

. The emitters 13 are supported at one end by means of T-shaped heads 15 interlocked with elongated apertures 16 in a filament ring 17 which is attached to the support member 9. The ring 17 is attached in insulated relation to a second ring 19 by means of bolts 21 and electrical insulation 22 and 23. Ring 19 is formed with apertures 20 aligned with the apertures 16 and grooves 11 and receiving one end of a plurality of bushings 24, the other ends of which abut one end of the shield element 10.

The other end of each of the emitters 13 has a T-shaped head 25 interlocked with a notch 26 in each of a plurality of flexible conductor supports 27 which are mounted on a ring 29 by bolts 31. The ring 29 is spaced from the shield element 10 by a plurality of bushings 33 mounted in apertures 35 provided in ring 29 in alignment with the grooves 11. The elements 19, 24, 10, 33 and 29 are brazed together in fluid-tight manner. The emitters 13 are held under tension by the resilient force of a plurality of leaf springs 37 which bear against the inner surfaces of the flexible conductor supports 27.

The spaces between the two rings 19 and 29 and the shield element 10 are closed externally in fluid-tight manner by two flanged rings 39. The inner opening in ring 29 is closed by a cup-shaped closure member 41 which extends into the central portion 3 as shown. The space within the shield element 10 and the flanged rings 39 is divided into two parts by a tubular partition member 43 and a pair of annular partition members 45 attached to the ends of member 43 and having apertures through which the bushings 24 and 35 extend. The end of the tubular partition 43 adjacent the ring 19 is connected, by means of an intermediate ring 47, to an inner fluid pipe 49 extending coaxially through the support member 9. The central opening in the ring 19 is connected to an outer fluid pipe 51 extending coaxially between the inner pipe 49 and the support member 9. It can be seen that the structure shown and described provides a continuous path for conducting a fluid cooling medium through the cathode, as indicated by the arrows shown in Fig. 1. It will be understood that the direction of flow may be reversed from that shown. By passing a cooling medium along the inner walls of the tubular shield element and the flanged rings 39 the heat generated in these elements by bombardment by electronsmay be dissipated, and the temperature of these elements may be maintained at any desired temperature.

In the operation of the magnetron, electrons leave the emitters 13 in all directions from the emitting surfaces. Preferably, the emitters are coated only on the surface facing the anode. In Fig. 2, several electron paths of the same curvature are indicated by arrows a, b, c, and r1. The'ettects on the electron paths of the induced high frequency fields associated with the anode elements 7 have been neglected, and hence, the actual electron paths are somewhat different from those shown. Arrows a and b repersent the paths of two electrons originating at opposite ends of an emitter 13 and returning to the cathode after just missing the anode elements 7. Arrows c and d represent the paths of electrons just missing the nearest rib 12 of the shield element 10. It will be noted that all of the returning electrons impinge upon electronopaque surfaces of the shield element 10, and none are able to bombard the emitting surfaces of the emitters 13. The two rings 39 serve as end shields or hats to confine electrons from the cathode to the space between the cathode and anode, in a manner known in the art. If desired, the outer edges of the shield ribs 12 may be formed with integral extension 57, to provide further protection for the emitters 13.

The cathode disclosed is designed for use in a multicavity magnetron having a continuous-wave output of two or three hundred kilowatts. The cathode will receive 15-20 kilowatts of back-bombardment power, corresponding to about 190-250 watts per square centimeter.

A somewhat similar magnetron cathode, in the form of a generally-cylindrical tubular member having outwardly-projecting longitudinal ribs with emitting coatings on one side face only of each rib, to prevent backbombardment of the emitting coatings by out-of-phase electrons, is disclosed and claimed in a copending application of Robert L. Sproull, Serial No. 654,934, filed March 16, 1946, now U. S. Patent No. 2,592,206, issued April 8, 1952, assigned to the same assignee. In the improved cathode of the present invention, the primary emitters are separated from the ribs to permit cooling the latter while heating the emitters.

What is claimed is:

l. A cathode comprising a shield element having a plurality of parallel V-shaped grooves, and an elongated flat ribbon-like thermionic emitter mounted in each groove closely adjacent, spaced from and in a plane parallel to the same side wall of each groove.

2. A cathode comprising a shield element having a plurality of parallel V-shaped grooves, and an elongated flat ribbon-like thermionic emitter mounted in each groove and spaced substantially closer to one side wall than to the other side wall of the groove and in a plane parallel to said one side wall, said one side wall of each groove being disposed at an angle of approximately 30 to a plane through the apex of the V and perpendicular to the surface of the shield element, and the other side wall of each groove being disposed at an angle of approximately 60 to said plane.

3. A magnetron cathode including an elongated hollow electron-opaque shield element of generally-cylindrical cross-section having a row of longitudinal grooves disposed around the periphery thereof, an elongated thermionic emitter located in each groove in position to be shielded from back-bombardment by electrons returning thereto at an acute angle to said row of the transverse magnetic field of a magnetron, each emitter being disposed closely adjacent to but spaced from the same side wall of each groove, and means for cooling said shield element comprising an elongated tubular partition coaxially mounted within said hollow shield element and forming two concentric passages, and two concentric tubular members connected to one end of said shield element and said partition for conducting a cooling fluid medium to and from said cathode.

4. A magnetron cathode including an elongated electron-opaque shield element of generally-cylindrical crosssection having a row of longitudinal grooves disposed around the periphery thereof, an elongated thermionic emitter located in each groove in position to be shielded from back-bombardment by electrons returning thereto at an acute angle to said row in the transverse magnetic field of a magnetron, each emitter being disposed closely adjacent to but spaced from the same side wall of each groove, a shield projecting radially outwardly from each end of said shield element, and means for cooling said shield element and said shields.

5. A cathode comprising a shield element having a row of outwardly-projecting parallel shield ribs forming the side walls of parallel grooves therebetween, each of said side walls extending at a substantial angle to said row, an elongated thermionic emitter disposed in each of said grooves in spaced relation to the walls thereof with the major portion of each emitter substantially closer to the same side wall, respectively, than the other side wall of the groove, and means for cooling said shield element.

6. A cathode according to claim 5, wherein each of said thermionic emitters comprises a flat emissive surface disposed in a plane parallel to said same side wall.

7. A cathode comprising an elongated shield, element of generally-circular cross-section having a plurality of outwardly-projecting longitudinal shield ribs uniformly disposed around the periphery thereof and forming the side walls of longitudinal grooves therebetween, each of said side walls extending at a substantial angle to said periphery, an elongated thermionic emitter disposed in each of said grooves in spaced relation to the walls thereof with the major portion of each emitter substantially closer to the same side wall, respectively, than the other side wall of the groove, and means for cooling said shield element.

8. A cathode according to claim 7, wherein each of said grooves is V-shaped in cross-section, and each of said emitters comprises a flat emissive surface disposed in a plane parallel to said same side wall.

References Cited in the file of this patent Journal of Applied Physics, vol. 18, No. 3, March 1947, PP. 314-320. 

